Olefin oligomerization

ABSTRACT

Limiting the sulphur level in olefin feedstocks to zeolite-catalysed oligomerization enhances selectivity to trimer and catalyst life and activity.

This invention relates to a process for the manufacture of highermolecular weight organic molecules from lower molecular weightmaterials, especially olefins, by oligomerization, to crystallinemolecular sieves suitable for use in the process, and the use of suchmolecular sieves in such reactions. The invention also relates to theoligomers produced and their use as feedstocks for further reactions.

Molecular sieve catalysts of many types have been proposed for use innumerous chemical processes. Among such processes are the conversion byoligomerization of lower olefins, e.g., alkenes, to higher olefins,e.g., higher alkenes, for example the oligomerization of C₂ to C₆,especially C₃ and C₄, olefins to olefins in the C₆ to C₁₂ range, andoccasionally higher.

An example of the use of crystalline molecular sieves as catalysts forthis purpose is given in EP-B-625 132, which describes the advantage ofusing in zeolite-catalysed olefin oligomerization, a hydrated feedstock,in particular one containing 0.05 to 0.25 molar percent of water.Another example is given in EP-B-746 538, where zeolites of thestructure types MFI, TON, and MFS, in their acid forms, are used inoligomerization of propene and butene, the particular members of thosestructure type families used being ZSM-5, ZSM-22, and ZSM-57. Thispatent is concerned with controlling the extent of oligomerization, toobtain the desired proportions of or selectivity to dimer, trimer, andhigher oligomers, for use in downstream manufacturing processes. In thepatent, methods of improving trimer yield are described.

Commercially available feedstocks have been found, however, to causedeactivation of the oligomerization catalyst, and to give a lowerselectivity to trimer than might be expected from the disclosure ofEP-B-746 538. Although the effects on activity can be countered to someextent by raising the reaction temperature, the catalyst life is reducedby this expedient.

The present invention is based on the observation that high levels ofsulphur compounds in the feed appear to be responsible for the problemswith commercial feedstocks.

The invention accordingly provides a process for the oligomerization ofan olefinic feedstock, which comprises contacting under oligomerizationconditions an olefinic feedstock containing from 0.1 to 2 ppm ofsulphur-containing compounds by volume with a catalyst comprising acrystalline molecular sieve and recovering a product comprising at leastone olefin oligomer.

More especially, the invention comprises reducing the content ofsulphur-containing compounds of an olefinic feedstock containing morethan 2 ppm by volume thereof to a level of from 0.1 to 2 ppm, beforecarrying out the oligomerization process.

Advantageously, the sulphur compound content of the feedstock is or isreduced to from 0.2 to 1 ppm by weight.

It has been found that certain types of sulphur-containing compounds areespecially deleterious. These are primarily saturated aliphaticcompounds, for example the thiols, sulphides, including cyclicsulphides, and disulphides. The low molecular weight materials areespecially troublesome, for example dimethyl, diethyl, and ethyl methylsulphides, n-propane thiol, 1-butane thiol and 1,1-methylethyl thiol,ethylmethyl and dimethyl disulphides, and tetrahydrothiophene. There mayalso be a steric factor, since n-propyl sulphide is more troublesomethan is di-isopropyl sulphide or isopropyl thiol. Aromatic compounds areless troublesome, as is carbonyl sulphide.

It has been found that sulphur compounds adsorbed or absorbed by, butnot readily desorbed thermally from, the catalyst are the mosttroublesome.

Accordingly, the invention also provides a process for theoligomerization of an olefinic feedstock, which comprises contactingunder oligomerization conditions an olefinic feedstock containing from0.1 to 2 ppm, advantageously from 0.2 to 1 ppm, by volume ofsulphur-containing compounds having a desorption temperature above 170°C. with a catalyst comprising a crystalline molecular sieve andrecovering a product containing at least one olefin oligomer.

More especially the invention provides reducing the sulphur compoundcontent of such a feedstock represented by such compounds to a level offrom 0.1 to 2 ppm, advantageously 0.2 to 1 ppm, by volume, beforecarrying out the process.

The sulphur-containing compound content of a feedstock is convenientlyascertained by gas chromatographic analysis using peak areas normalizedwith reference to a COS standard.

The crystalline molecular sieve is advantageously of the structure typeTON or MFS, the term “structure type” being used in the sense describedin the Structure Type Atlas, Zeolites 17, 1996. Examples of TONstructure type zeolites include ZSM-22, ISI-1, Theta-1, Nu-10, and KZ-2,and of MFS include ZSM-57, all in their H— or acid form.

The crystalline molecular sieve is advantageously ZSM-22 or, preferably,ZSM-57. ZSM-22 and its manufacture are described in, for example, U.S.Pat. No. 4,556,477 and WO 93/25475, and ZSM-57 and its manufacture inEP-A-174 121 and U.S. Pat. No. 4,973,870, the disclosures of all ofwhich are incorporated herein by reference. Mixtures of two or moremolecular sieves may be used, e.g., a mixture of ZSM-22 and ZSM-57.

Although ZSM-22 and ZSM-57 are presently preferred, there may also beused a crystalline molecular sieve of the MTT structure type,exemplified by ZSM-23, EU-13, ISI-4 and KZ-1.

A molecular sieve crystallite size advantageously up to 5 μm, preferablywithin the range of from 0.05 to 5 μm, more especially from 0.05 to 2μm, and most preferably from 0.1 to 1 μm, may be employed. The molecularsieve may be supported or unsupported, for example in powder form, orused as an extrudate with an appropriate binder. An as-synthesizedmolecular sieve is advantageously converted to its acid form by acidtreatment, e.g., by HCl, or by ammonium ion exchange, and subsequentcalcination before use in the process of the invention. The calcinedmaterials may be post-treated as by steaming. Although the inventionwill be described with reference to zeolites proper, it is possible touse, as is known in the art, a material in which silicon and aluminiumhave been replaced in whole or in part by other elements, silicon moreespecially by germanium and aluminium more especially by boron, gallium,chromium and iron, materials containing such replacement latticeelements also being termed zeolites, and the term is used in the broadersense in this specification, to include aluminophosphates andsilico-aluminophosphates (AlPO's and SAPO's).

The olefin feedstock advantageously contains olefins having from 2 to 12carbon atoms, preferably from 2 to 6 carbon atoms, and is advantageouslyan alkene-containing feedstock. The feed preferably contains propene,butenes and/or pentenes; the invention is especially applicable topropene oligomerization.

Sulphur-containing compounds may be removed from the feedstream by anymethod, many of which are used in industry or described in theliterature. As examples, there may be mentioned washing the feed with aheavy hydrocarbon, e.g., Isopar L, contacting the feedstream withchloramine-T, as described in U.S. Pat. No. 5,167,797, or use of a guardbed, to absorb sulphur either physically, with for example a molecularsieve, e.g., Z10-08 (trade mark) from Zeochem, or chemically, using anickel catalyst. Substantially complete sulphur absorption was achievedat 190° C. using a nickel catalyst.

Reaction conditions for the process of the invention may be, with theexception of the presence of the sulphur compound or compounds, inaccordance with conditions operative for prior art processes foroligomerization of the same olefin or olefins.

The olefinic feedstock may be fed to the reaction zone in the liquid or,preferably, the supercritical phase. The feedstock may contain water,either present from the feedstock raw material or by addition.

The feedstock advantageously comprises from 0.05 to 0.25, preferablyfrom 0.06 to 0.20 and more preferably from 0.10 to 0.20, molar % waterbased on the total hydrocarbon content of the feedstock. If desired orrequired, the natural water content of the feedstock may be increased,for example, by being passed through a thermostatted water saturator.Since the amount of water required to saturate the feedstock will dependupon the temperature and composition of the feedstock, control of thewater content may be effected by appropriate control of the temperatureof the feedstock.

The feedstock may also comprise an inert diluent, for example, asaturated hydrocarbon. That other hydrocarbon is included in thehydrocarbon content for the purposes of calculation of the watercontent.

The reaction is advantageously carried out at a temperature within therange of from 130 to 300° C., preferably at a temperature within therange of from 135° to 280° C., more preferably from 160° to 250° C., andmost preferably from 160° to 230° C. It will be appreciated that, tomaintain desirable conversion rates, it may be advantageous to increasereaction temperatures with the time the catalyst is on stream.Preferably, however, the temperature is kept below 230° C. whenpossible, to prolong catalyst life.

The pressure is advantageously in the range of 5 to 10 MPa, preferablyfrom 6 to 8 MPa. The olefin hourly space velocity is advantageously inthe range of from 0.1 to 20, preferably from 1 to 10, and morepreferably from 1.5 to 7.5, hr⁻¹.

As indicated above, it was found when using a commercial olefinicfeedstock containing more than 2 ppm sulphur-containing compounds byvolume, especially a propene-containing feed, that in addition toreduced catalyst activity, the selectivity to trimer on oligomerizationwas lower than expected from the disclosure of EP-B-746 538. It has beenfound that by limiting the proportion of sulphur in the feed,selectivity to trimer is improved. Similarly, with limited sulphur inthe feed, oligomerization may be performed at lower temperatures,thereby prolonging catalyst life.

The invention accordingly also provides the use of limiting to from 0.1to 2 ppm, and advantageously from 0.2 to 1 ppm, by volume, theproportion of sulphur-containing compounds in an olefin feedstock beingfed to a crystalline molecular sieve catalyst to maintain or enhance (a)selectivity to trimer production or (b) catalyst activity or to maintainor enhance both (a) and (b). Further, since to achieve a givenconversion rate with a sulphur-containing feed a higher operatingtemperature is required, and higher temperatures cause more rapidcatalyst ageing, a lower sulphur proportion has a beneficial effect oncatalyst lifetime.

The invention accordingly further provides the use of limiting to 0.1 to2 ppm, and advantageously from 0.2 to 1 ppm, by volume, the proportionof sulphur-containing compounds in an olefin feedstock being fed to acrystalline molecular sieve catalyst to maintain or enhance catalystlife.

The sulphur compounds so limited may be those with a desorptiontemperature above 170° C.

As indicated above, the oligomers of the invention are especiallysuitable as feedstocks for further processing, including at least one ofthe following: fractionation; hydrogenation; hydroformylation;oxidation; carbonylation; etherification; epoxidation, and hydration.

The eventual products may be alcohols, produced for example byhydroformylation and hydrogenation; esters, in which the alcohols areesterified as with inorganic or organic acids, including carboxylicacids, especially polycarboxylic acids; aldehydes, acids, in which thehydroformylation products are oxidized and hydrogenated, and numerousother end uses.

The esters with polycarboxylic acids are especially valuable asplasticizers, and the invention further provides plasticizercompositions comprising the esters, and polymeric compositionsespecially vinyl chloride polymers, particularly PVC, comprising theesters, and shaped structures formed of the plasticized polymericcompositions.

EXAMPLES

The following examples, in which parts and percentages are by weightunless otherwise indicated, illustrate the invention.

All feeds used in the examples were hydrated by passage through a watersaturator at 25 to 40° C. The feeds are as follows:

Feed A—commercial propene feed, containing 48% propene, 3% butenes, 4%ethane, 20% propane, 9% n-butane, 16% isobutane and 22.4 ppmsulphur-containing compounds by volume (3 ppm thiophenes, 2 ppmmercaptans (thiols), 10 ppm dialkyl sulphides, 3 ppm dialkyldisulphides, 1 ppm H₂S, the remainder unidentified sulphur-containingspecies).

Feed B—feed A after sulphur removal by guard bed (sulphur compoundcontent <0.1 ppm by volume).

Feed C—sulphur-free (sulphur compound content <0.1 ppm by volume)propene feed, containing 50% propene, 40% n-butane, 10% isobutane.

-   -   Catalyst compositions used are as follows:    -   Catalyst A—an extruded H-ZSM-57 catalyst    -   Catalyst B—a powdered sample of H-ZSM-57    -   Catalyst C—an extruded H-ZSM-22 catalyst.

Olefin monomer conversion rates were derived from gas chromatographicanalysis using peak areas normalized to the total sum of the paraffinsin the feed as internal standard, conversion being expressed as:${{conversion}\quad\%} = {100\left\lbrack {1 - \frac{A\quad{o.m.\text{/}}A\quad{paraffins}}{{A{^\circ}}\quad{o.m.\text{/}}{A{^\circ}}\quad{paraffins}}} \right\rbrack}$where A represents chromatographic peak area in product (wt %), A°represents chromatographic peak area in feed (wt %) and o.m. representsolefin monomer(s).

Selectivity to a given oligomer (dimer, trimer, etc.) is also determinedfrom gas chromatographic peak areas, after hydrogenation of the productstream.

Example 1

The specified feeds were passed over catalysts A and B, ZSM-57, at atotal feed space velocity of 2 h⁻¹, the temperature being increased from135° C. until an alkene conversion rate of at least 80% is reached. Theconversion rate and selectivity to dimer were analysed, and the resultsare shown in Table 1. TABLE 1 Jacket Nonene Sample Feed Temp ° C.Catalyst Conv. % Selectivity C1 C 135 B 95 74 C2 C 165 A 95 72 1 C + 1ppm 160 B 94 63 CH₃SH 2 C + 1 ppm 215 A 95 63 CH₃SH C3 C + 7 ppm 228 A96 61 CH₃SH C4 B 142 B 96  60* C5 A 225 B 95  53**Selectivity lower than Sample C1 because of co-oligomerization with thebutene content of the feed.

The results show that selectivity to nonene is adversely affected by thepresence of sulphur-containing compounds.

Example 2

To various samples of Feed C were added various sulphur compounds beforeoligomerization over Catalyst B. The initial conversion rate, thedecrease in conversion rate with time on stream, and the selectivity tononenes were observed. The results are shown in Table 2. TABLE 2Compound, content ppm Conversion After (expressed as ppm by Initial DaysOnstream, % Nonene Desorption volume) Conversion % 1 2 4 5 Selectivity %Temp ° C. <0.1 95 93 90 87 75 — Isopropyl Sulphide, 2 93 92 90 85 75 145Thiophene 2 95 93 91 88 75 Ethylsulphide 7, thiophene 95 88 41 3 60 5,tetrahydro thiophene 2 Dimethyldisulphide 2 95 72 5 65 180diethylsulphide 7 94 100 4 64 >210 Approximation to 95 58 5 59commercial feed Tetrahydrothiophene 2 95 90 85 33 47 180 DimethylSulphide 2 92 91 41 61 195 n-propylthiol 94 90 84 70 72 160isopropylthiol 2 98 94 90 83 76 160 COS 5 90 91 88 81 73 170

1-22. (Cancelled)
 23. A process for the oligomerization of an olefinicfeedstock, which comprises contacting under oligomerization conditionsan olefinic feedstock containing from 0.1 to 2 ppm sulphur-containingcompounds by volume with a catalyst comprising a crystalline molecularsieve and recovering a product comprising at least one olefin oligomer.24. A process as claimed in claim 23, which comprises reducing thesulphur content of an olefinic feedstock containing more than 2 ppmsulphur-containing compounds by volume to a level of from 0.1 to 2 ppmby weight before carrying out oligomerization.
 25. A process for theoligomerization of an olefinic feedstock, which comprises contactingunder oligomerization conditions an olefinic feedstock, containing from0.1 to 2 ppm sulphur-containing compounds by volume in compounds havinga desorption temperature above 170° C., with a catalyst comprising acrystalline molecular sieve and recovering a product containing at leastone olefin oligomer. 26 A process as claimed in claim 25, whichcomprises reducing the sulphur content of an olefin feedstock containingmore than 2 ppm sulphur-containing compounds represented by suchcompounds to a level of from 0.1 to 2 ppm before carrying out theoligomerization.
 27. A process as claimed in claim 23, wherein thecrystalline molecular sieve is of the TON structure type.
 28. A processas claimed in claim 27, wherein the crystalline molecular sieve isH-ZSM-22.
 29. A process as claimed in claim 23, wherein the crystallinemolecular sieve is of the MFS structure type.
 30. A process as claimedin claim 29, wherein the crystalline molecular sieve is H-ZSM-57.
 31. Aprocess as claimed in claim 23, wherein the crystalline molecular sievecomprises a mixture of H-ZSM-22 and H-ZSM-57.
 32. A process as claimedin claim 23, wherein the crystalline molecular sieve is in powder formor in the form of an extrudate.
 33. A process as claimed in claim 23,wherein the feedstock comprises at least one olefin having from 2 to 12carbon atoms.
 34. A process as claimed in claim 33, wherein thefeedstock comprises at least one alkene selected from propene andbutenes.
 35. A process as claimed in claim 23, wherein thesulphur-containing compounds content of the feedstock is from 0.2 to 1ppm.
 36. A process as claimed in claim 23, wherein thesulphur-containing compound comprises at least one compound selectedfrom dimethyl sulphide, diethyl sulphide, ethyl methyl sulphide,n-propyl sulphide, n-propanethiol, 1-butanethiol, 1,1-methylethylthiol,ethyl methyl disulphide, dimethyl disulphide and tetrahydrothiophene.37. A process as claimed in claim 23, wherein the feedstock is hydrated.38. A process as claimed in claim 23, wherein the process is carried outat a temperature of at most 300° C.
 39. A process as claimed in claim23, wherein the process is carried out at a temperature within the rangeof from 135° to 280° C.
 40. A process as claimed in claim 23, whereinthe process is carried out at a temperature within the range of 160° C.to 250° C.
 41. An olefinic oligomeric mixture produced by the process ofclaim
 23. 42. An ester derived from the olefinic mixture of claim 41.43. A plasticized polymeric composition containing the ester as claimedin claim 42.